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How did our forebears dry lumber? Question
If not, what did they do differently that we can't do today? And are we really going to encounter problems with air dried lumber as many people indicate we will? Maybe the old timers air dried their lumber first and then brought it into their working environment for a sufficient period of time to acclimate the wood before using it. Maybe this made the difference. Anyone have any historical information on this? Forum Responses
I think that the main reason air dried lumber worked in the 18th century is because of the lack of air conditioning and heating. So, most likely the homes probably had conditions close to the conditions the lumber dried in to begin with. I'm by no means a carpenter, but I couldn't see myself putting all this time and craftsmanship into a piece of furniture only to watch it go unsquare and develop cracks in the joints.
The last theory is correct--in earlier times the lack of what we would consider adequate heating and cooling played a big role in not only the necessary moisture content, but also in the design. The air exchange in a current day home is almost nil as compared to one from prior centuries, and when coupled with humidifiers and dehumidifiers, we enjoy far greater stability in humidity and temperature. Air exchange would be responsible for greater swings in moisture content especially season to season, as well as a higher MC in winter months because of moisture being able to enter the living space more readily. Because of this, their designs had to be capable of adjusting to a greater range of humidities. A more recent example of this would be wooden single pane windows. They were difficult to operate in the summer because of swelling due to higher humidities and drafty and loose fitting in the winter due to reduced humidities because of heating. Winter typically sets the low MC requirement because air that is cold is denser (let's say compressed) and when heated it expands (decompressing), creating a greater volume: the water molecules in the air are now spread out over a greater volume and thus the amount of water present in a given volume of air is less than it was before it was heated, hence the term relative humidity. Also, there is greater area between the air molecules allowing more water to be held. The real key to remember in drying is balance. The moisture content in the wood must balance with that of the air, neither gaining nor losing any significant amount of moisture. Fortunately, the wood will reach that balance for us. Unfortunately, its time table doesn't usually fit into ours, thus requiring kilns to create artificial conditions that don't normally exist year round (extremely high temperatures with extremely low humidities), allowing us not only to dry wood faster, but to be able to acheive MC levels comparable to those only found in winter months. If you lived in a greenhouse "dry" would be quite different compared to the desert Southwest. All that is required is balance, not a magical number or percentage, to have functional wood. You can reach the same MCs as kiln dried wood by air drying--it is merely a function of time and conditions (and sweat).
The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment). Comment from contributor J:
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